Title : Mount a folder on another folder
Author: Solène
Date  : 22 May 2018
Tags  : openbsd66 openbsd

This article will explain quickly how to bind a folder to access it
from another path. It can be useful to give access to a specific
folder from a chroot without moving or duplicating the data into the
chroot.

Real world example: "I want to be able to access my 100GB folder
/home/my_data/ from my httpd web server chrooted in /var/www/".

The trick on OpenBSD is to use NFS on localhost. It's pretty simple.

    # rcctl enable portmap nfsd mountd
    # echo "/home/my_data -network=127.0.0.1 -mask=255.255.255.255" > /etc/exports
    # rcctl start portmap nfsd mountd

The order is really important. You can check that the folder is
available through NFS with the following command:

    $ showmount -e
    Exports list on localhost:
    /home/my_data               127.0.0.1

If you don't have any line after "Exports list on localhost:", you
should kill mountd with `pkill -9 mountd` and start mountd again. I
experienced it twice when starting all the daemons from the same
commands but I'm not able to reproduce it. By the way, **mountd** only
supports reload.

If you modify */etc/exports*, you only need to reload **mountd** using
`rcctl reload mountd`.

Once you have check that everything was alright, you can mount the
exported folder on another folder with the command:

    # mount localhost:/home/my_data /var/www/htdocs/my_data

You can add `-ro` parameter in the */etc/exports* file on the export
line if you want it to be read-only where you mount it.

Note: On FreeBSD/DragonflyBSD, you can use `mount_nullfs /from /to`,
there is no need to setup a local NFS server. And on Linux you can use
`mount --bind /from /to` and some others ways that I won't cover here.